The invention relates to a device and method for sieving foreign bodies out of a material stream in the course of the production of oriented strand boards. Furthermore, the invention relates to an apparatus or system for the production of composite wood boards with such a device.
In the large-scale industrial production of composite wood boards continuously operating presses are used. In these presses, such as those described in DE 39 13 991 C2, the pressing force is transferred by hydraulic actuating elements onto the pressing or heating plates and further, via steel belts which are disposed so as to be supported via a bed of rolling bodies (roller rods) and so as to be endlessly revolving onto the material to be pressed. In the case of said transfer elements the steel belts with thicknesses of around 1.5 to 4 mm form, in comparison to the other, significantly stiffer elements, are the weakest component in the chain of machine elements and are prone to failure. In this connection this does not mean that the steel belt fails completely. Should the surface of the steel belt have defects, this already leads to an inadequate surface quality of the end product and thus to rejection. In the production of ever thinner, and above all highly compacted, composite wood boards the deformation work to be applied increases so sharply with partial differences in the density of the material to be pressed that even variations in density that have a very small surface area, e. g., lumps of adhesive in the material to be pressed, can lead to pressure marks on the surface of the steel belt or even to destruction of the steel belt. For the large-surface hydraulic cylinders in use in the main press area such partial variations in pressure lie outside of the measurable area, in particular since still more machine elements are disposed between the steel belt and the hydraulic cylinder. Thus it is not possible to recognize and, if necessary, correct denting of the steel belts which arises via measuring devices on the large hydraulic cylinders.
In the course of the development of the technology there has been in recent years increasing research, and poor results, in the use of detection systems for determining elevations in density in order to remove lumps of adhesive or similar foreign bodies of harmful size from a spread mat of material to be pressed before it is pressed. Usually this happens by ejecting the corresponding portion of the mat from the forming belt into a discard hopper. Since, however, the recovery of poorly spread pressed material mat or trial spreadings is the usual task of the discard hopper, it can happen that detected foreign bodies get into the production cycle once again since a complete sieving of all the material out of the discard hopper once again is very expensive.
Basically there is at present no technological solution for production systems which make it possible, at an acceptable cost, to filter the lumps of adhesive greater than 15 mm in diameter out of a material stream consisting of flakes to which adhesive has just been applied. Traditional sieve systems are not suitable for this purpose since they provide the necessary minimum throughput per hour only with large-scale sieve systems which are expensive to procure and to maintain. Also these sieve systems, which usually screen in such a manner that thinner or smaller material falls through the sieve and larger material is discharged at the end, have the disadvantage that lumps of adhesive can “hide” in the larger material and continue to remain in the material stream.